Studies of vasopressin-stimulated water and solute transport across the isolated toad bladder have shown that the two transport systems appear to be under separate control, especially in relation to the pathways that become available to water and solutes in the luminal membrane. We are able to activate the water and solute pathways separately, by carefully controlling the concentrations of vasopressin or cyclic nucleotides used in stimulating the bladder. Employing this technique, we are studying the membrane morphologic changes involved in water or urea transport, the role of cAMP-dependent kinase in the two transport systems, and the site of action of inhibitory agents such as the prostaglandins, on the cyclic nucleotide - kinase sequence.